Review

Graphene-based light sensing: fabrication,characterisation, physical properties and performance.

Adolfo De Sanctis1,‡, ID , Jake D. Mehew1,‡, ID , Monica F. Craciun1 ID and Saverio Russo1,* ID

1 Centre for Graphene Science, College of Engineering, Mathematics and Physical Sciences, University ofExeter, Exeter EX4 4QL, United Kingdom;* Correspondence: s.russo@exeter.ac.uk;‡ These authors contributed equally to this work.

Received: ; Accepted: ; Published:

Abstract: Graphene and graphene-based materials exhibit exceptional optical and electrical propertieswith great promise for novel applications in light detection. However, several challenges preventthe full exploitation of these properties in commercial devices. Such challenges include the limitedlinear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation andextraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects,large-scale fabrication and ultimately the environmental stability of the constituent materials. In orderto overcome the aforementioned limits, different approaches to tune the properties of graphene havebeen explored. A new class of graphene-based devices has emerged where chemical functionalisation,hybridisation with light-sensitising materials and the formation of heterostructures with other 2Dmaterials have led to improved performance, stability or versatility. For example, intercalation ofgraphene with FeCl3 is highly stable in ambient conditions and can be used to define photo-activejunctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable andversatile material which supports the photodetection from UV to THz frequencies. Nanoparticles andquantum dots have been used to enhance the absorption of pristine graphene and to enable high gainthanks to the photogating effect. In the same way, hybrid detectors made from stacked sequencesof graphene and layered transition-metal dichalcogenides enabled a class of detectors with highgain and responsivity. In this work we will review the performance and advances in functionalisedgraphene and hybrid photodetectors, with particular focus on the physical mechanisms governingthe photoresponse in these materials, their performance and possible future paths of investigation.

Keywords: Graphene; graphene oxide; photodetectors; sensors; functionalisation; electronic devices.

1. Introduction

The discovery of graphene [1] and more broadly of atomically thin materials has triggered a wealthof research in optoelectronics [2], plasmonics [3], telecommunications [4], solar energy harvesting[5] and sensing [6]. Several novel applications in these sectors exploit the unique combination ofbroadband absorption, ultrahigh ambipolar mobility and field effect tunability inherent to single layergraphene [7] and its compatibility with unconventional substrates such as recent developments intextile electronics [8,9]. However, the lack of a bandgap and the intrinsic low light absorption of thissingle layer of carbon atoms poses some challenges for its use in practical applications [10]. Chemicalfunctionalisation [11] has been proposed as a route to engineer an energy gap in the energy dispersionof graphene. At the same time, the ability to combine different two-dimensional (2D) materials into vander Waals (vdW) heterostructures [12] has transformed this field of research owing to the possibility tocreate clean interfaces among system with very diverse physical properties such as semiconductors,insulators, superconductors, magnetic materials and ferroelectrics to list a few [13].

Photodetectors are used in nearly every electronic device which interfaces with the external worldor with other devices. Several industrial sectors make use of light sensors such as telecommunications,food production, transport, defence and healthcare. Although the miniaturisation of electronic devices

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such as transistors has allowed higher computational speeds and smaller, portable devices, theminiaturisation of light sensors did not proceed at the same rate, as several physical factors limitthe scaling of such devices. In particular, the realisation of ultra-thin and flexible photodetectors isparticularly challenging with conventional semiconductor technologies, due to the brittle nature of thematerials used and low absorption at nano-scale thickness. Graphene-based light sensors have shownsome exceptional performances, spanning from high speed [4] to large linear dynamic range (LDR) [14],and they offer transparency and flexibility for future applications in wearable electronics [8]. In thiswork, we will review the progress in the fabrication and characterisation of photodetectors (PDs) usingchemically functionalised forms of graphene and hybrid graphene heterostructures with nanoparticles(NPs), transition-metal dichalcogenides (TMDs) and organic crystals. Whilst functionalisation canbe used to efficiently modify the charge carrier dynamics in graphene which in return can lead toenhanced photoresponse, the hybridisation with NPs, TMDs and organic semiconductors boosts theabsorption of light, therefore increasing the efficiency of the PDs. After a description of the materialsand fabrication techniques, we will focus our attention on the main physical mechanisms responsiblefor photodetection in these materials. We will then review the most relevant papers which demonstratetheir performance, highlighting strong and weak points for each device, as well as their suitability inspecific applications.

2. Materials and fabrication of graphene-based photodetectors

Graphene can be obtained via different methods. The first and most direct approach ismicro-mechanical cleavage of bulk graphite [7]. This method gives very high quality single- andmulti-layer graphene flakes, with very high values of mobility and low defect densities. Exfoliatedgraphene is often the starting material for functionalisation or for the creation of hybrids andheterostructures. However, this method is not scalable and has a low throughput. More scalablemethods to produce graphene have been developed with chemical vapour deposition (CVD) beingthe most promising technique to produce large-area graphene [15]. Depending on the substrate, CVDgraphene can be grown as a single-layer or as a multi-layer. In its multi-layer form, CVD graphene iswell suited to intercalation. Another scalable technique is liquid-exfoliation of graphite to producegraphene dispersion in water or other solvents [16]. This route is often used to produce multi-layergraphene depositions, for example using vacuum filtration. Other production techniques includeepitaxial growth on silicon carbide [17] and reduction of graphene oxide (GO), a functionalised formof graphene.

Chemical functionalisation refers to the use of chemical species to modify the properties of amaterial and in graphene it can take different forms [11], which are illustrated in figure 1a. These are(1) the intercalation of chemical species between the layers of graphene, as with FeCl3 [18], (2) thesubstitution of a carbon atom with an atom of a different specie or molecules, as with graphene oxide[19], or (3) the chemisorption of chemical species to saturate the π bonds, as with fluorographene [20].The functionalisation routes described can be realised using both solution- (e.g. sol-gel, hydrothermal,hydrolysis, solvent/ion exchange) and dry-process methods (e.g. plasma sputtering, annealing,thermal evaporation, CVD and PVD). Different techniques result in different size of the material,different defects and contaminants as well as different functionalization. For example, graphite oxideis prepared in solution, typically following Hummers’ method, with GO isolated through liquid-phaseexfoliation of the bulk material. Such a method is able to sustain a high production throughputas opposed to the oxgyen plasma treatment of CVD grown graphene, also known to produce GO[21]. Each type of functionalisation produces materials with different electronic properties, spanningfrom metallic [18,22,23] to a wide-bandgap insulator and magnetic systems [17]. A functionalisedgraphene PD is generally made by contacting the active material with metal contacts, either defined bylithography or through the use of a shadow mask. Sometimes encapsulation is necessary due to theenvironmental instability of some of the constituent materials. This is usually achieved by depositing a

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S1 S2

∆TPTE

CNPEF

EPV PG

EF

VBM

CBmSensitising

material

Graphene

Fe

Intercalation Oxidation Fluorination

OCl H F

QD

Graphene

TMDs

Graphene

vdW gap

e-

h+

a

c

b

Figure 1. Materials and photodetection mechanisms in graphene-based devices. (a) Examples offunctionalised graphene materials. (b) Examples of hybrid/heterostructure graphene materials:quantum dots (QD) and van der Waals (vdW) heterostructures made with graphene andsemiconducting TMDs. (c) Three main mechanisms responsible for photo-activity in graphene-baseddevices: photothermoelectric effect (PTE), photovoltaic effect (PV) and photogating (PG). S1, S2

represent Seebeck coefficients, T is the temperature, CNP is the charge neutrality point, EF it theFermi level, E is the electric field, VBM is the valence band maximum and CBm is the conduction bandminimum.

polymer layer on top, such as Poly-methyl-methacrylate (PMMA), or by using an ionic liquid [24–26]such as lithium perchlorate-PEO (LiClO4-PEO) which also acts as a gating electrode [27,28].

A multitude of techniques can be employed in order to form hybrids and heterostructures betweengraphene and other materials. These include coating techniques such as spin/spray/dip/cast/barcoating, printing techniques such as inkjet and contact printing and deposition methods such asthermal evaporation, CVD, electrochemical deposition, etc. However, the majority of these methodsresult in a certain degree of defects being introduced in the graphene material. In this review, wefocus our attention onto non-destructive techniques which allow the realisation of heterostructureswhich preserve the high-mobility of graphene field-effect transistors (FETs) and enhance the lightabsorption of the device. For this reason we will consider the following two techniques: spin-coating(or equivalent deposition method) of nanoparticles or quantum dots (QDs) directly on the surfaceof graphene and stacking different 2D materials in a van der Waals (vdW) assembly [12], as shownin figure 1b. In both cases charges are extracted from the graphene layer by means of metal contacts.Encapsulation of vdW heterostructures is usually achieved using hexagonal Boron Nitride (hBN)[29], PMMA or sputtered oxides such as SiO2 or AlO2. Encapsulation in hBN allows the formationof one-dimensional contacts [30] (also known as side-contacts), which have been proved to give thelowest contact resistance whilst preserving the intrinsic properties of graphene resulting in recordhigh charge carrier mobility, without any high temperature annealing steps common to high qualitygraphene devices.

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Table 1. Summary of parameters used to characterise PDs. Coupling Factors and wavelengthdependence of all quantities are omitted for clarity of notation.

Quantity Symbol Definitiona UnitsExternal Quantum Efficiency ηe, EQE (Iph/q)/φin %Internal Quantum Efficiency ηi, IQE (Iph/q)/φabs %Operating Bandwidth ∆ f - HzGain G (µτE) /L -Responsivity R Iph/Popt A/W (V/W)Noise Equivalent Power NEP Sn/R W/

√Hz

Specific Detectivity D? (A∆ f )0.5/NEP cm√

Hz/WLinear Dynamic Range LDR 10× log10 (Psat/NEP) dB

a Iph = Measured photocurrent, φin = Incident photon flux, φabs = Absorbed photon flux, Popt =φin/S = Incident optical power density, A = Device area, Sn = Noise spectral density, Psat = Saturationpower, L = channel length, τ = lifetime of photoexcited carriers, E = electric field;

In the context of materials for light-sensing applications, it is important to notice the differencebetween functionalisation, which refers to changes in the structure or nature of the host material, andhybridisation, which refers to property combination of two or more materials. The former resultsin a new material which is directly used as both light-absorber and charge-transport layer. In thelatter, generally, one material acts as light-absorber, whilst the other, graphene in this case, acts ascharge-transport layer.

3. Light detection in graphene-based devices

The main different mechanisms responsible for the photoresponse of functionalised and hybridgraphene PDs can be grouped in three categories: photothermoelectric (PTE), photovoltaic (PV) andphotogating (PG) effects. These three mechanism are schematically shown in figure 1c. They all rely onthe creation of a non-equilibrium distribution of photo-excited carriers and their diffusion or drift in apotential gradient. Although they can all be present in one device, one of them is dominant dependingon the geometry and the microscopic carriers dynamic. Such dominant effect dictates the performanceof the photodetector and its range of technological applicability.

Other three mechanisms responsible for photodetection in graphene devices are the bolometriceffect [31], the Dyakonov-Shur effect [32] and plasmon-assisted photocurrent generation [33]. Theseeffects are particularly suited to detect mid-infrared (MIR) to THz radiation, however they will notbe considered in this review as they are not dominant mechanisms in the functionalised and hybridgraphene PDs under consideration. Other methods to enhance the absorption of pristine graphene,such as coupling to plasmonic structures [34,35], wave-guiding [36] and micro-cavity resonators [37]will not be considered since these techniques rely on engineering the substrate or the device ratherthan modifying the active material. For an overview on how nanophotonics structures are used ingraphene photodetectors we suggest the review by Xia et al [38] and references therein.

3.1. Characterisation and figures of merit

The basic characterisation techniques rely on shining light onto the device whilst recording itselectrical response. Light can impinge on the whole surface of the device, known as flood illumination,or it can be delivered with a focused laser onto a specific area to allow for a spatially-resolvedphoto-response, such as in scanning photocurrent mapping (SPCM) [39]. Both techniques give insighton the physical nature of the observed photoresponse. With these techniques it is possible to extractthe key quantities which define the performance of a photodetector, which are summarised in table 1.

The responsivity R = Iph/Popt is defined as the ratio between the measured photocurrent (orphotovoltage) Iph = I − Idark, where Idark is the dark current, and the incident optical power Popt

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and it is measured in units of A/W (V/W). Noise in photodetectors plays an important role inreal-life applications, the main figure of merit for the characterisation of noise is the Noise EquivalentPower (NEP), defined as the incident power necessary to produce a signal-to-noise ratio of 1 at 1 Hzbandwidth. It is given by the noise spectral density Sn divided by the responsivity: NEP = Sn/Rand measured in units of W/

√Hz. The bandwidth ∆ f of a PD is defined as the frequency at which

its output power drops by 1/2, that is when the photocurrent drops by ∼ 70.7% (known as −3 dBbandwidth). These quantities are used to define the main figure of merit in PDs performance, thespecific detectivity D∗ = (A∆ f )0.5 /NEP, where A is the area of the device. D∗ is measured in Jones(cm√

Hz/W) [40]. The linear dynamic range (LDR) determines the region of incident power withinwhich the photodetector has a linear response [14]. It is defined as the logarithm of the ratio betweenthe saturation power Psat (at which the response of the detector deviates from linearity) and the NEP:LDR = 10× log10 (Psat/NEP). A final figure of merit is the gain which depends on the mobility µ, thephotoexcited carriers lifetime τ and the applied electric field E: G = (µτE) /L.

Figure 2 and table 2 show a summary of the performance of the devices considered in this review.In general, in a plot of responsivity vs. bandwidth, we can see a net separation between functionalisedgraphene and hybrid PDs, with the former having lower responsivity values than the latter (shadedareas in figure 2a). However, large LDR is observed in some functionalised graphene detectors, albeitwith low values of responsivity. High LDR and high responsivity are both found in hybrid graphenePDs, thanks to the low NEP found in such devices. In terms of spectral response, figure 2c showsthat both type of detectors are suited to a very wide range of incident photon energy. Of particularrelevance is the ability of GO-based PDs to operate from UV to THz frequencies.

3.2. Photothermoelectric effect

A junction between two materials with different Seebeck coefficients S1 and S2, in which the twosides are held at different temperatures, is subject to a voltage, known as thermoelectric voltage [41]. Ingraphene, absorbed photons create a population of carriers with an increased temperature with respectto the surrounding. Such temperature gradient ∆T, in the presence of a boundary to a material with adifference in Seebeck coefficient ∆S = S1 − S2 (see figure 1c), causes a photovoltage to be generated:

∆V = ∆T · ∆S, (1)

where the signs is dictated by either gradients. This is known as photothermoelectric effect (PTE). TheSeebeck coefficient can be expressed using the Mott relation[42,43]:

S =2π2kBTe

3qTF, (2)

where Te is the electron temperature, TF = EF/kB is the Fermi temperature, q is the electron chargeand kB is the Boltzmann constant. Equation 2 assumes that the mobility does not depend on theFermi energy EF, i.e. at relatively large doping. For EF ' 0.1 eV the Seebeck coefficient has a valueS ∼ 0.1 mV/K [44]. In general, the amount of energy taken by the hot carriers is given by ChTh ∝ Popt,where Ch is the heat capacity and Popt is the incident optical power. Assuming the hot carriersthermalise at a temperature far above that of the lattice, their specific heat is Ch ∝ T2

h and, combiningequations 1 and 2, the generated photocurrent is: IPTE ∝ T2

h . Therefore the proportionality between thegenerated photocurrent and the incident optical power is:

IPTE ∝(

Popt) 2

3 . (3)

The exponent in equation 3 is commonly measured in graphene photodetectors, however a rangeof other exponents are possible due to PTE, depending on the dominant cooling mechanism. Inparticular an exponent of 1 is possible if the electronic temperature is only marginally above the latticetemperature [14].

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Wavelength (µm)0.2 0.3 0.4 0.5 0.7 2 10 150

G

FeCl3

GO

Other

0.6 0.8

NP2D

Organ.

0.9 4 6 8 30Perov.

UV Vis NIR MIR FIR THz

Bandwidth (Hz)10-4 10-2 100 102 104 106 108

Res

pons

ivity

(A/W

)

10-8

10-6

10-4

10-2

100

102

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101010-10

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5

10

15

20

25

30

35

40

45

100

LDR

(dB

)

120

107 109

a b

c

Figure 2. Graphene-based photodetectors performance comparison. (a) Responsivity vs bandwidthand (b) LDR vs responsivity in functionalised (filled symbols) and hybrid (open symbols) graphenePDs. Shaded areas encircle the majority of points belonging to each group. (c) Operational wavelengthrange for different graphene-based PDs, points correspond to experimentally tested wavelengths, linesrepresent full spectral scans. Detailed data in table 2, reference numbers in brackets. NP = nanoparticles,2D = TMDs/heterostructures.

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It has been shown that the PTE is responsible for the light-sensing ability of pristine grapheneand that the photo-active areas are confined to the junctions between two different materials suchas the graphene/metal [4,45,46], monolayer/bilayer graphene [47–49] and graphene/functionalisedgraphene interfaces [50]. Furthermore, the type of substrate and gate dielectric in graphene FETshas been shown to change the PTE properties of pristine graphene devices [51]. The hot-carrierdynamic in graphene has been extensively studied [52–55] and the ability of graphene to generatelarge PTE voltages related to its unique band structure and high Fermi velocity (vF ' 106 m/s) whichlimit the available states in the Fermi sphere for acoustic phonon scattering (responsible for cooling).Consequently, this limits the energy dissipation for photo-excited carriers, creating a population ofelectrons with a large effective temperature. Although PTE has been shown to dominate in graphenejunctions devices, the presence of photovoltaic (PV) effects cannot be excluded, as it has been recentlyreported in single/multi-layer graphene junctions [56].

3.3. Photovoltaic effect

The term "photovoltaic effect" has been commonly used by the solar cell research community todescribe a broad variety of mechanisms by which the absorption of photons, generation of excitons,separation into free charge carriers and collection of free charge carriers at electrodes sequentiallytakes place. This definition is quite broad and does not necessarily require the presence of an electricfield. However, within the research field of atomically thin materials the PV effect refers to the processof separation of photo-generated carriers by a built-in electric field [2,45,57,58]. These charges aresubsequently extracted by a diffusion process in the short-circuit configuration or accelerated by anapplied electric field to the electrodes. In this case, the resulting photocurrent is equal to [59]:

Iph =ηiPopt

hω0

WL

τcq (µe + µh)Vsd, (4)

where ηi is the internal quantum efficiency (IQE), hω0 is the energy of the photon, W and L are thewidth and length of the device, τc is the carriers lifetime, µe and µh are the electron and hole mobilitiesand Vsd is the applied bias. Equation 4 shows that the observed photocurrent has a linear dependencewith respect to the incident optical power: Iph ∝ Popt.

3.4. Photogating and gain mechanism

To increase the absorption of graphene-based photodetectors a semiconducting material is placedin close proximity to the channel of a graphene field-effect transistor (FET). Upon illumination aphotoexcited charge carrier is transferred from the semiconductor to graphene. This changes thecarrier density in the graphene FET which manifests itself in electrical measurements as a shift in thecharge neutrality point (VCNP) - effectively a photo-activated gate, hence the name photogating effect(PG).

Such a system can be treated as a photoconductor with distinct light-absorbing andcurrent-carrying regions. The photocurrent (Iph) flowing in a device of area A = WL and thickness Dis described by [60]:

Iph = (σE)WD = (qµnE)WD, (5)

where σ is the conductivity, E the electric field across channel and µ the mobile carrier mobility. Withthe following definition for the number of photogenerated carriers (n);

n =η(Popt/hν)τ

WLD, (6)

which includes the number of incident photons (Popt/hν), quantum efficiency (η) and recombinationrate (1/τ). By using the earlier definition of responsivity (R = Iph/Popt) we arrive at:

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R =( q

hν

)η

(µτE

L

)=

( qhν

)ηG. (7)

The responsivity of a typical hybrid graphene photodetector depends on three terms: the first iscomprised of physical constants whilst the second and third terms relate to the quantum efficiencyand gain of the system respectively, both of which need to be maximised.

Light will be absorbed by a semiconductor if the incident photons have energy greater than theband gap (hν ≥ Eg). In this case electron-hole pairs are generated which form an exciton with anintrinsic efficiency (ηgen) that relates to the absorption coefficient of the material. To create free charges,the Coulomb force between electron and hole must be overcome. This can happen under the influenceof large electric fields or due to thermal energy and this process has an associated efficiency term (ηdiss).Charges are transferred between semiconductor and graphene in the presence of a potential barrier atthe semiconductor-graphene interface or from a charge trapping mechanism in the semiconductor.In addition clean interfaces are required for efficient charge transfer (ηtrans). Therefore the quantumefficiency can be split into three terms:

η = ηgenηdissηtrans. (8)

Applying a bias voltage to the graphene channel allows the transferred charge to be extractedat the drain contact. To preserve electrical neutrality a charge must be simultaneously injected at thesource. This process of charge recirculation can occur multiple times until the recombination of thetrapped charge takes place. Such a process is described by the gain term in equation 7. To achieve thelargest gain the ratio between the trapped carrier lifetime (τ) and free carrier transit time (ttr = L/µE)must be maximised: G = τ/ttr.

Long-lived charge trapping is achieved by the spatial separation of photoexcited charges acrossthe interface. τ limits the photodetector response time and as such there is a trade-off between gainand bandwidth. To minimise the transit time a high mobility channel, short electrode spacing and largeelectric fields are desirable. Graphene is the most promising material to achieve the unique situation inwhich an ultra-high carrier mobility can be accessed at the surface with micron scaled devices [61]readily fabricated using standard electron-beam lithography techniques.

4. Functionalised graphene photodetectors

In this section we will review some of the main functionalisation strategies used to enhancephoto-detectivity in graphene-based devices, with particular focus to intercalation with ferric chloride(FeCl3) and the use of graphene oxide (GO) as the two main forms of graphene functionalisationin which PDs with exceptional performances and scalability have been demonstrated. Otherfunctionalised graphene PDs, such as those based on fluorographene (FG), will also be reviewed.

4.1. FeCl3-intercalated graphene photodetectors

The intercalation of graphite with different chemical species is a well-known route to modify itsbulk properties [63]. More recently, few-layer graphene (FLG), i.e. between 2 and 5 layers, has beenemployed as a novel platform to exploit the intercalation technique. For instance, intercalation offew-layer graphene with ferric chloride (FeCl3-FLG) results in a new material with enhanced opticaland electrical properties [18]. The strong charge-transfer between graphene and FeCl3 molecules [64]induces large p-doping of graphene [22], up to 1014 cm−2, and drastically changes the carriers dynamics[65]. The intercalation also results in a superior transparent conductor with a sheet resistance as low as8 Ω/sq with 85 % transparency [18] highly sought for sensing and efficient lighting technologies [66].Contrary to bulk graphite, the intercalation of FLG takes place at relatively low temperatures using athree-zones furnace, it does not require a carrier gas and the time-scale is reduced from tens of days to

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P P' P

EF

ECNP

Iph

A

LASER FeCl3-FLG c

b

d

n tot (

1014

cm

-2)

1.71.92.12.3

1.51.3

r (μm)-10 -5 5 100

PristineIrradiated

3μm-9

0

9

-9

0

9

Iph (nA)

Pristine

Irradiated Iph (nA)

A

Vbg

SiO2

Si

Au

FeCCl

λin

Laser

Iph5μm

nh5μm

-60

-200

20

60

0 3 6 9 12 15

4

5

I ph (n

A)

n h (10

13 c

m-2)

r (μm)

nh

a

b

c

d

Figure 3. FeCl3-FLG Photodetectors. (a) Laser-defined p-p’ junction in FeCl3-FLG device, ECNP is thecharge neutrality point and EF is the Fermi level. (b) Scanning photocurrent (Iph) maps before (top) andafter (bottom) laser-irradiation along the white dashed lines. Red dashed lines delimit the FeCl3-FLGflake, laser wavelength was 375 nm. Adapted and reprinted with permission from De Sanctis et al[14], under CC-BY license from AAAS, 2017. (c) Schematic of a multi-terminal hexagonal-domainFeCl3-FLG photodetector and measurement circuit. (d) Photocurrent (Iph, top) and charge density(nh, bottom) extrapolated from the corresponding maps (right panels); Green shaded areas representmaxima and minima of the photocurrent which correspond to a change in charge density. Reproducedwith permission from De Sanctis et al [62], under CC-BY license from IOP Publishing Ltd, 2017.

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Table 2. Summary of key performance parameters for graphene, functionalised graphene and hybrid PDs. LDR and D∗ values are reported only if availablefrom the experimental data. Range of R, ∆ f , D? and LDR are given corresponding to the range in ∆λ.

Ref. Type/Functional. Response R (A/W) ∆ f (Hz) D? (Jones) ∆λ (nm)a LDR (dB)Pristine graphene

[4] Interdigitated PTE 6.1 · 10−3 1.6 · 107 6 · 105 b 1500 7.5[70] Suspended PTE/PV 6.25 · 10−4 10 1.3 · 104 b 540 24[45] Dual-gated PTE 1.55 · 10−3 − − 532 −[71] Log-antenna PTE 5 · 10−9 7 · 109 − 30(µm)− 220(µm) −

Functionalised graphene

[14] FeCl3 PV (0.015− 0.1) · 10−3 700 103 b 375− 10000 44[62] FeCl3 PV 0.1 · 10−3 − − 375 −[72] GO/rGO PV 0.12 1.6 · 10−4 − 360 −[73] GO/rGO PV 4 · 10−3 3 · 10−2 − 1550 −[74] GO/rGO PV 2.4 · 10−4 − 1.4 · 10−3 2− 2.5 − 375− 118.6(µm) 7− 11[75] 3D np-rGO PV 1.33 · 103 − 1.13 · 104 6 · 10−4 − 370− 895 4[76] GO/Na2So4 PV (17.5− 95.8) · 10−3 2− 50 · 10−3 − 455− 980 −[77] GO PV 1 · 10−3 − 1 · 10−6 2.2 3 · 107 375− 1610 25[78] GO PV 1.6 · 10−7 − 1.8 · 10−6 7 · 10−3 − 1064 −[79] rGO/ZnO PV 1 · 10−7 − 3 · 10−7 3.3 − 532− 1064 11[80] rGO/TiO2 PV − 0.1 − > 400 −[81] FG PG 1000− 10 3 4 · 1011 − 1 · 109 255− 4290 4[82] BTS/ATS SAMs PTE 0.02 100 − 532 15

QDs, Organics and heterostructures

[83] PbS QDs PG 5 · 107 10 7 · 1013 600− 1750 30b

[84] PbS QDs PG 1 · 106 1.2 − 895 −[85] ZnO QDs PG 1 · 104 − − 325 −[86] ZnO QDs PG 1 · 104 0.07 5.1 · 1013 335 36 b

[87] ZnO QDs PG 2.5 · 106 − − 326 −[88] CdS NPs PG 4 · 104 1000 1 · 109 349 −

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[27] CdSe/CdS NPs PG 10 10 106 532− 800 −[89] PbS QDs/ITO PG/PD 2 · 106 4 · 103 1 · 1013 635− 1600 110[90] Si QDs PG 0.1− 2 · 109 − 103 − 1013 375− 3900 −[91] PbS QDs/MAPbI3 PG 2 · 105 100 5 · 1012 400− 1500 24[92] MAPbI3 PG 18− 180 4 1 · 109 400− 1000 −[93] MAPbBr2I PG 6 · 104 2.9 − 405− 633 −[94] MAPbI3 + Au NPs PG 2.1 · 103 0.2 − 532[95] MAPbI3 PG 1.7 · 107 0.4 2 · 1015 b 450− 700 −[96] Chlorophyll PG 1.1 · 106 0.78 − 400− 700 −[97] Ruthenium PG 1 · 105 0.125 − 450 −[98] P3HT PG 1.7 · 105 5.8 − 500 −[99] C8-BTBT PG 1.6 · 104 14 − 355 −[100] Rubrene PG 1 · 107 0.014 9 · 1011 400− 600 −[101] MoS2 PG 5 · 108 − − 635 −[102] MoS2 PG 1 · 109 − 1 · 1012 609 −[103] MoS2 PG 1 · 107 − − 650 −[104] MoS2 PG 46 − − 642 −[105] GaSe PG 4 · 105 35 1 · 1010 532 −[106] MoTe2 PG 970 4.5 1.6 · 1011 1064 −[28] WS2 PG 1 · 106 1500 3.8 · 1011 400− 700 12[107] Tunnel barrier PG 1.1− 103 35 − 532− 3200 −

a Unless other units specified; b Values calculated from the published data;

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only 8 hrs. This, together with its environmental stability [67] and its peculiar plasmonic properties[68], make FeCl3-FLG the ideal candidate for novel opto-electronics devices [69].

Indeed, the ability to selectively intercalate graphene led to the engineering of its photo-response.As shown in figure 3a, a laser beam can be used to control the microscopic arrangement of FeCl3molecules by selectively de-intercalating FeCl3 from the graphene layers. In this way, a photoactivep-p’ junction can be defined, as shown in the scanning-photocurrent maps (SPCM) in figure 3b.These laser-defined junctions have been shown to have an extraordinary LDR of 44 dB, the highestreported in an all-graphene device (see also table 2). Furthermore, the spectral responsivity of suchdevices was also maintained, as they have been demonstrated to operate from ultra-violet (UV) tomid-infrared (MIR) wavelengths (see figure 2c). The key of this performance relies on the carriersdynamic engineered in these junctions. In graphene, the LDR is limited by the PTE effect and thelimited density of states (DOS) available for photo-excited carriers. Since both PTE and PV effects cancontribute to the observed photoresponse, careful analysis of the resulting power dependencies (seeequations 3 and 4) and the direction of the observed photocurrent demonstrate that the in FeCl3-FLGthe response is dominated by PV effects, whilst PTE effects are strongly quenched [14]. For example,hot-carriers dynamic in graphene prevent its use in high-resolution sensing due to the smearingof the photoactive region up to several tens of microns [45]. However, by quenching such effectsin laser-defined junctions in FeCl3-FLG it is possible to overcome this limitation and surpass thediffraction-limit of far-field microscopy, as demonstrated using near-field techniques. In this wayphotoactive regions with a peak-to-peak distance of 250 nm, (less than half the laser wavelength used)were fabricated. Such nano-scale photoactive junctions hold the promise for the realisation of noveldevices in biomedical applications [108].

FeCl3-FLG has also been used to realise an all-graphene position-sensitive photodetector (PSD)[62]. In this case a CVD-grown hexagonal domain of bilayer graphene [109] was used as startingmaterial and intercalated following the same procedure used in previous works [17,18,22]. Amulti-terminal geometry, arranged along the edges of the hexagons, allows to measure photocurrentbetween different pairs of opposing contacts (see figure 3c). Strikingly, the observed photocurrentdisplayed a bipolar and monotonic behaviour in regions in which the intercalation-induced chargedensity changes abruptly. These changes were shown to be related to the partial intercalation of theFLG along specific lines irradiating from the centre of the hexagons (figure 3d). Therefore, photoactivep-p’ junctions were formed in the hexagonal crystal at the growth stage. This work was the firstdemonstration of position-sensitive behaviour in an all-graphene PD (i.e. where both the active andtransport layers are made of graphene).

The responsivity of FeCl3-FLG PDs is in line with other pristine graphene devices (0.1− 1 mA/W)and it is limited by the absence of a gain mechanism able to multiply the photo-generated carriers inthe material. However, the suppression of PTE increases the operating bandwidth of these detectorsup to 700 Hz, when compared to other functionalised graphene PDs (such as GO) this is two to fourorders of magnitude higher. In FeCl3-FLG PDs the speed is limited by the diffusion time of the excitedcarriers which is affected by the reduced mobility due to the high levels of doping [18,22,110].

4.2. Graphene oxide

A functionalised form of graphene decorated with oxygen atoms (in the form of carboxyl, hydroxylor epoxy groups) is graphene oxide (GO) [19,111]. GO is an insulator in which the energy bandgapand electrical conductivity can be tuned by reducing its oxygen content in what is known as reducedgraphene oxide (rGO) [112]. GO is usually prepared in solution and it is a scalable way of obtaininggraphene-based materials [113], composites [114] and devices for microelectronics [115]. Furtherfunctionalisation is possible in GO and rGO by replacing oxygen groups with other chemical species[76] or by interfacing GO with other nano-structured materials such as nanoparticles [80].

The photoresponse of GO has been investigated by many authors in devices with differentdegrees of reduction and functionalisation. Solution-processed GO has been used to demonstrate

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Figure 4. GO and rGO Photodetectors. (a) Fully suspended rGO photodetector grown on Si nanowirearray and (b) photoresponse of this photodetector under UV and THz illumination. Reprinted withpermission from Yang et al [74], Carbon 115, 561-570, Copyright 2017 Elsevier. (c) Fabrication stepsof 3D nanoporous rGO (3D np-rGO). (d) Power-dependent responsivity for two sample 3D np-rGOdevices at λ = 370 nm. Reprinted with permission from Ito et al [75], Adv. Func. Mater. 26, 1271.Copyright 2016 John Wiley and Sons. (e) Schematic of TiO2 nanoparticles embedded in GO matrix. (f)Cathodic photoresponse of GO/TiO2 photodetectors under visible illumination, on and off indicate theswitching of the light source. Reproduced with permission from Chen C. et al [80], ACS Nano 2010 4,6425. Copyright 2010 American Chemical Society.

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photodetection across a wide spectral range (see figure 2c). Chitara et al [72] demonstrated thatchemically-reduced GO solution [116] can be used to realise a UV-sensitive photodetector with aresponsivity of 120 mA/W operating at λ = 360 nm. Soon after, the same authors demonstratedthat a similar photodetector can operate at infra-red (IR) wavelengths (λ = 1550 nm) [73] althoughwith a reduced responsivity of 4 mA/W. In both cases the operating bandwidth of these deviceswas of the order of 0.1− 30 mHz, corresponding to rise and fall times of several seconds. The worksof Chitara et al show that GO PDs can operate across an exceptionally large range of wavelengths.Recently, Yang et al [74] demonstrated a free-standing rGO photodetector able to operate from UV(λ = 375 nm) to THz (λ = 118.6 µm) wavelengths (see figure 4a-b) with responsivity ranging between0.24 mA/W and 1.4 mA/W (see table 2) and an operating bandwidth of 2− 2.5 Hz. This technologyis characterized by a LDR in the range 7− 11 dB. The ease of fabrication and the scalability of thematerial make this kind of suspended rGO photodetectors very attractive for macroelectronics. Otherauthors compared the photoresponse of GO and rGO photodetectors. Chang-Jian et al [78] preparedGO and rGO photodetectors form GO solution and demonstrated that photocurrent from rGO deviceswas due to the separation of excited electron-hole pairs (PV-type of response), where electrons areinjected into the contact at higher potential (positive), thus giving an increase in photocurrent. Onthe contrary, in GO devices a cathodic photocurrent was observed, which can be attributed to theinjection of holes into the negative contact due to the work-function mismatch between the GO andthe Au contacts. Such detectors, when illuminated with IR light (λ = 1064 nm) showed responsivity of1.8 µA/W and operating bandwidth of 7 mHz.

Qi et al investigated the response of solution-processed GO in the Visible-IR range. By drop-castingGO solution onto a glassy carbon electrode they were able to measure the photoresponse of thismaterial in Na2SO4 solution. Their experiments report responsivity values of 95.8− 17.5 mA/W forλ = 455− 980 nm with an operating bandwidth of 2− 50 mHz. In the same work they demonstrate UVsensitivity under λ = 280− 350 nm illumination, although the data doesn’t allow the estimation of theresponsivity at this wavelengths. The solution-processed approach is very scalable and attractive forapplications in the chemical industry where PDs can be used to work in specific solutions. However,these detectors were found to be unstable in ambient conditions and degraded under UV illumination.

Low responsivity in GO devices is attributed to the poor electrical contact between stackedflakes, which is a result of solution-processing techniques, and the reduced light absorption due tothe 2D nature of the individual flakes. Furthermore, the large amount of defects in GO and rGOfilms limits the charge carriers mobility and increases charge recombination sites, limiting the overallefficiency of the device. In order to improve the responsivity of GO photodetectors, Ito et al [75]developed a device based on 3D nanoporous rGO (3D np-rGO, see figure 4c). This material displaysand enhanced absorption in the near-UV region (λ = 300− 400 nm) indicating a large density ofstates at high photon energies. However, the long absorption tail indicates that defect states allowphoton absorption across the UV-Visible and IR range. Indeed, the photoresponse of np-rGO wasdemonstrated to strongly depend on the reduction time and to reach a maximum responsivity of1.13× 104 A/W at λ = 370 nm and 1.33× 103 A/W at λ = 895 nm for samples reduced for 150 min.However, as it can be seen in the power-dependence of the responsivity in the UV range (see figure4d), such high values of responsivity are measured at very low power densities (0.01 µWcm−2) andrapidly decreases by more than three orders of magnitude as the power density reaches 300 µWcm−2.Furthermore, the saturation of the responsivity at higher power suggests that the device is operatingabove the NEP on this region only, giving an estimate for the LDR of ∼ 4 dB. The behaviour of thepower-dependent responsivity suggests that the photocurrent mechanism is that of a photoconductor,whereby illumination creates a population of electron-hole pairs which are separated by an appliedelectric field across the semiconductor. The operating bandwidth can be estimated from the measureddecay time and equals 6 mHz, in line with other GO-based photodetectors.

In order to improve the performance of GO PDs and to add functionalities such as photocatalyticproperties, several groups worked on preparing composite materials combining solution-processed

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GO and rGO with oxide nanoparticles such as TiO2 or ZnO. Chen et al [80] demonstrated a GO/TiO2

composite which can be used as a photodetector (see figure 4e). In their experiments they foundthat both a cathodic and anodic photoresponse was possible depending on the starting concentrationof GO in the initial solution (see figure 4f). This indicates a different doping of the GO compositedepending on the amount of TiO2 in the initial solution (see figure 4e). The photoresponse wasobserved at wavelengths > 400 nm across the Visible-NIR spectrum. Interestingly, this compositematerial displayed photocatalytic properties when illuminated with wavelengths > 400 nm, as shownby the degradation of methyl orange. Liu et al [79] developed a rGO/ZnO nanowire PD. In this case,the hybrid PD behaves like a photodiode, rather than a photoconductor. The responsivity of thisdevice is in line with others in the range 0.1− 0.3 µA/W and it is able to operate across the visible-IRspectrum (λ = 532− 1064 nm) with a LDR of 25 dB and an operating bandwidth of 6 mHz. The PVresponse of this hybrid PD was demonstrated to be due to the Schottky junction between the ZnOnanowires and the GO, which also accounts for the relatively low responsivity.

The pursuit of an environmentally friendly method to produce electronic materials led to researchinto ways to make graphene and GO from many sources. Lai et al [77] demonstrated a verticaljunction photodetector made from GO produced by processing of glucose solution [117]. This devicestructure comprises two layers of GO prepared with different annealing temperatures sandwichedbetween an ITO and a Au electrode. Such device is shown to operate from deep-UV (λ = 290 nm)to IR (λ = 1610 nm) across the whole visible spectrum. They report responsivity values between1 mA/W and 1 µA/W going from UV to IR with a LDR of 25 dB at (λ = 410 nm). The operatingbandwidth of this device (see table 2) is in the range 1− 6 Hz, in line with other reports on GO PDs.The photoconductive behaviour of this PD and the linearity of the power-dependent photocurrent (seeequation 4) allow to conclude that the PV photocurrent generation is responsible for its photo-activity.

From the data reported in literature, it is clear that all GO-based PDs have a very slow responsetime. This can be attributed to the intrinsic defective nature of the active material. Charge traps inthe GO act as pinning centres for photoexcited electron-hole pairs and allow both fast recombination,which limits the responsivity, and slow carrier drift/diffusion, which limits the speed [118]. However,the presence of these defects states and the gapless nature of rGO allow to absorb light across awide spectral range, spanning from deep-UV to THz (see figure 2), albeit with large variations inresponsivity. In particular, the large sensitivity in the UV region makes GO and rGO PDs extremelypromising for replacing current semiconductors in applications such as environmental monitoring,water purification and defence [119].

4.3. Other functionalised graphene PDs

Similar to GO, the attachment of fluorine atoms to graphene results in another insulating form offunctionalised graphene, known as fluorographene (FG) [120]. As with rGO, the degree of fluorinationin FG can be changed during fabrication [121] or afterwards using, for example, e-beam irradiation[122,123]. The ability to selectively tune the insulating properties of FG gives a versatile material forphotodetector technology. Du et al [81] investigated a FG/graphene photodetector where graphene isused as a charge transport layer and FG as the charge-trapping layer, creating a photogating-based (PG)device (figure 5a). Fluorination modifies the C-C bond hybridisation from sp2 to sp3. These confinedareas have different charge trapping times, while the pristine graphene layer acts as high-mobility layer,enabling charge re-circulation and high gain (see equation 7 and figure 5b). This photodetector worksover a broad range of wavelengths, from UV (λ = 255 nm) to MIR (λ = 4.29 µm), albeit MIR rangewas tested only at a temperature of 77 K. Responsivity varies across the spectrum, from a maximumof 103 A/W in the UV to 10 A/W in the MIR range, and with incident optical power (see figure 5c).Indeed, a drop in the responsivity as a function of power indicates saturation of the photocurrent, asshown in figure 5c. From the data provided (figure 5d) we can estimate a LDR of 4 dB and an operatingbandwidth of 3 Hz, in line with the high responsivity values and the slow response given by the trapstates in the FG. Overall, the performance of the device varies with the degree of fluorination (i.e. C/F

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Figure 5. FG Photodetectors. (a) Device schematic of a FG/graphene PD. (b) Domain structure ofpartially-fluorinated graphene and corresponding lifetimes of photoexcited charges (left). Trapping andrelaxation schematic in FG responsible for the observed photo-response. (c) Responsivity as a functionof incident power density for multiple wavelengths and (d) comparison of NEP as a function of incidentwavelength for a graphene/graphene and a FG/graphene device. Reprinted with permission from Duet al [81], Adv. Mater. 29, 1700463. Copyright 2017 John Wiley and Sons.

ratio) and it is observed to be maximum for all wavelengths for a C/F ratio of 3.5− 3.75. Indeed,another study highlighted the role of the density of defect states in the photoresponse of FG/graphenephotodetectors [124]. Statistical analysis of several samples also shows a small sensitivity to fabricationvariables since the responsivity is observed to vary within a factor of 1.5 at maximum. Interestingly,given the different trapping time in the sp2 and sp3 regions, the authors demonstrate that it is possibleto extract the different contributions using AC+DC modulation of the incident light, where the DCcontribution to the photocurrent is attributed to the sp3 sites (slow traps) and the AC response to thesp2 ones (fast traps). The ratio between the charge trapping times allows to estimate a photoconductivegain of 2× 105, weakly dependent on the wavelength.

A different approach to graphene functionalisation was followed by Wang et al [82]. In this workthey present a p-n junction graphene PD realised using CVD graphene on top of silane-modified SiO2.Self-assembled monolayers (SAMs) of 3-aminopropyltriethoxysilane (ATS) and N-butyltriethoxysilane(BTS) where put in contact with graphene to achieve p- and n- doping in adjacent regions. SPCMrevealed photocurrent generation at the interface between these two regions. Responsivity of theorder of 0.02 mA/W and an operating bandwidth of 100 Hz were achieved in the visible range. Thephotoresponse of the device was measured to be due to PTE, enabled by the difference in Seebeckcoefficient between the n- and p- region. Power-dependent photocurrent allows to estimate a LDRof 15 dB for this device. The use of SAMs and CVD graphene makes this technique scalable and thedevice performance is promising for sensing and imaging applications. However, the environmentalstability and bio-compatibility of the silane-based SAMs have not been tested.

5. Hybrid and heterostructure photodetectors

The inherent low responsivity of graphene-based devices is related to its small absorption perlayer and to the lack of a mechanism able to multiply the photogenerated carriers. Therefore themaximum quantum efficiency of a pure graphene device cannot exceed 1. Such limit can be overcome

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if a gain mechanism is present (equation 7). One way to attain this is to combine graphene with aphotoactive material and use the high mobility of the graphene channel to extract one photoexcitedcarrier, enabling charge recirculation. We will consider different light-sensitizing materials used ingraphene-based PDs, including quantum dots (QDs), perovskites, organics and TMDs.

5.1. Graphene/Quantum Dots and perovskites interfaces

Quantum Dots (QDs) are highly suited as a light-sensitizing material for graphene due to strongabsorption from UV to NIR, size tunable band gap and low temperature deposition [125]. Oftenthese are synthesised in solution and spin coated onto a target substrate. Such processing means thatproduction can readily be scaled up and non-destructively applied [126]. This is particularly importantfor graphene as conventional material deposition processes are known to induce defects and disorder[127].

The first report of a hybrid graphene-quantum dots photodetector is the work of Konstanatos et al[83]. A two terminal graphene FET was spin coated with an 80 nm thick film of lead sulphide (PbS)QDs with the first exciton peak at 950 nm or 1450 nm, see figure 6a. Photogenerated holes transferto graphene whilst electrons remain in the QDs. A built-in field at the graphene/QD interface andcharge traps within the PbS QDs keep the electron trapped for a time-scale τ. The transferred holeschanges the conductivity of the graphene channel leading to a shift in the Dirac point voltage, figure6b. Owing to the high-mobility of graphene the hole can be recirculated multiple times resulting in again that exceeds 108 electrons/photon, yielding a responsivity of ∼ 107 A/W. The charge trappingmechanism limits the bandwidth to 10 Hz, however recombination can be accelerated by providing anelectrical pulse to the gate. This lowers the built-in field allowing the trapped electron to recombinewith the hole. This strategy reduces the temporal response to∼ 10 ms. Similar device performance wasachieved using CVD graphene which represents an important step in the future commercialisation ofsuch detectors [84]. To further increase the response time Nikitskiy et al deposited an ITO top contacton the PbS-QD film [89], thereby incorporating a photodiode into the phototransistor structure. Thistransformed the passive sensitising layer into an active one increasing the charge collection efficiencyclose to 100% when surface reflections are taken into account (> 70% without). Applying a smallvoltage ∼ 2 V to the ITO increases the depletion region at the graphene-QD interface increasing thecontribution to charge collection from carrier drift over diffusion. They reported two major changesin device performance compared to the passive example [83]. The bandwidth is increased to 1.5 kHzas the carrier lifetime is now limited by the time constant of the ITO-QDs-graphene photodiode asopposed to the charge trapping lifetime. A significant enhancement in LDR to 110 dB was reported,representing the highest value achieved in hybrid graphene photodetectors, figure 6d.

Other QDs have been used including Zinc Oxide (ZnO) which has a much stronger absorption inthe UV than visible [85,86]. Shao et al demonstrated a UV phototransitor by coating graphene withZnO QDs [86]. A self-assembled monolayer (SAM) was deposited on the SiO2/Si substrate priorto the graphene transfer resulting in an increased mobility due to a reduction in charged impurityscattering. They reported a gate-tunable responsivity of 4× 109 A/W (at λ = 335 nm) for the devicewith a UV-Visible rejection ratio of ∼ 103. The response time exceeds 2 s which is likely due to theoxygen mediated charge-trapping mechanism (figure 6c) [85,86]. Ni et al used the localized surfaceplasmon resonance of doped Silicon QDs to enhance the MIR absorption of a graphene phototransistor[90].

Robin et al [27] have engineered a photodetector based on the charge-transfer at the interfacebetween graphene and CdSe NPs (CdSe-np). Such NPs, in their pristine form, display sharp excitonicpeaks which makes them ideal for wavelength-selective PDs. In their work they use epitaxially-growngraphene on silicon carbide (SiC) onto which a colloidal solution of CdSe-np is drop-casted and thenembedded into an ionic polymer gate. Photogating (see figure 1c, right panel) is the key mechanismwhich is responsible for photo-activity in this device: excitons created in the nanoparticles are splitand one of the carriers is injected into the graphene depending on the local doping, whilst the other

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Figure 6. Quantum Dots and Organic graphene hybrids. (a) Schematic of PbS QD sensitised graphenechannel with electrical connections. (b) Back-gate dependence of graphene channel resistance underdifferent illumination powers demonstrating photogating effect. Inset shows two-dimensional plotof power and back-gate voltage. Reproduced with permission from Konstantatos et al [83] Copyright2012, Nature Publishing Group. (c) Plateau in responsivity vs optical power shows the extended LDRof ITO-QD-graphene phototransistor. Reproduced under CC-BY 4.0 from Nikitskiy et al [89], 2016. (d)Oxygen assisted charge-trapping process in ZnO QDs. Reproduced with permission from Shao et al[86] Copyright 2015 American Chemical Society. (e) Photos taken in dense fog of a cylindrical objectby a conventional silicon (upper) and PbS-graphene hybrid (lower) sensor array. Reproduced withpermission from Goossens et al [128] Copyright 2017, Nature Publishing Group. (f) Photoluminescenceof rubrene single crystal with (blue) and without (red) graphene underlayer. Inset shows optical imageof device where coloured dots highlight position where the PL spectra were acquired. Reproducedwith permission under CC-BY 4.0 from Jones et al [100], 2017.

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remains trapped in the CdSe-np. This process gives an enhanced responsivity if compared to pristinegraphene of 10 A/W with an operating bandwidth of 10 Hz. Furthermore, the broadband absorptionof graphene combined with the excitonic features of the CdSe-np allow this device to operate acrossthe visible and NIR range (λ = 532− 800 nm).

Asides from QDs, perovskites have shown promise in the next generation of hybrid graphenephotodetectors with ideal properties such as direct band gap, large absorption and relatively highmobility [92–95]. However, the reported values for responsivity range from 102 to 107 A/W with thespread in values likely due to the poor coverage and uniformity of the perovskite films. In additionperovskites are highly unstable in environments containing oxygen and water.

Recently the first image sensors based on graphene photodetectors have been reported [91,128]. These combine sensitised graphene with a complementary metal-oxide-semiconductor (CMOS)readout circuit through back-end-of-line integration. This results in a sensor capable of broadbandlight-detection (UV-NIR) - previously unobtainable in a monolithic CMOS detectors. One particularadvantage is shown in figure 6e, where the graphene sensor reveals an object shrouded by fog thatwould otherwise be invisible to a conventional Si-CMOS sensor.

5.2. Graphene/organics interfaces

Organics have also been investigated as light-sensitising elements in graphene FETs. They are ofparticular interest owing to their intrinsic affinity to biological systems as well as the ability to tailortheir spectral selectivity through chemical functionalisation. A number of different materials havebeen investigated including chlorophyll [96], ruthenium [97], P3HT [98], and C8-BTBT [99]. Howeverthe reported quantum efficiencies are inferior to those of QDs (∼ 25%) [83]. Disorder in the crystalstructure could play a major role as evidenced by amorphous P3HT exhibiting an efficiency ∼ 0.002%[98] whilst for polycrystalline C8-BTBT 0.6% [99] was achieved. In the work of Jones et al singlecrystals of rubrene were laminated onto CVD graphene channels [100]. The long-range herringbonestacking of rubrene molecules in a single crystal allows exciton diffusion over several micrometers withrubrene exhibiting strong photoluminesence (PL) in the visible. However when placed in contact withgraphene the PL intensity is suppressed by ∼ 25%, see figure 6f. This suggests that a large numberof excitons dissociate at the graphene-rubrene interface. Indeed this hybrid photodetector exhibits aresponsivity as large as 107 A/W and an internal efficiency above 5% thanks to this charge transferand charge recirculation, as shown in figure 6g.

5.3. Graphene/TMDs vdW Heterostructures

The family of 2D materials extends beyond graphene to include insulators (hBN), semiconductors(e.g. MoS2 and WS2), and materials exhibiting more exotic properties such as superconductivityand magnetism [13]. Similar to graphene adjacent layers are separated by a van der Waals (vdW)gap facilitating the rapid prototyping of devices by mechanical exfoliation of bulks flakes. Recentlytechniques have been developed to create complex heterostructures though layer-by-layer assemblyof 2D flakes [12]. Naturally, the semiconducting 2D materials present an ideal sensitising layer ingraphene photodetectors due to their strong-light matter absorption and visible to NIR band gap [129].

The first demonstration of a graphene-TMD phototransistor was found in the work of Roy et al[101]. Here, graphene was placed onto a few-layer flake of molybdenum disulphide (MoS2), see figure7a. Electron-hole pairs are photoexcited within the MoS2 and one type of charge carrier transferred tothe graphene channel. Due to the formation of localized states at the MoS2/SiO2 interface persistentphotocurrent is observed (no return to initial dark state) indicating an extremely long trapped carrierlifetime. With the ultra-high mobility of the graphene channel (µ = 104 cm2V−1s−1 this translatesto a gain of 5× 1010 electrons/photon and responsivity of 5× 108 A/W. Using this structure theauthors demonstrated a multilevel optoelectronic memory using light and gate pulses to write anderase each bit respectively, figure 7b. In a follow up work they replaced monolayer graphene withbilayer and by dual gating they electrostatically opened a band gap of ∼ 90 meV [102], which resulted

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Figure 7. TMD-graphene hybrids. (a) Schematic of MoS2-graphene photodetector with scanningelectron microscope (SEM) image of device (lower inset) and principle of operation (upper inset).(b) Temporal response of photocurrent from device in (a) under illumination and gate pulse cycles.Reproduced with permission from Roy et al [101] Copyright 2013, Nature Publishing Group. (c)Histogram of normalized current shift indicating number resolved photon counting. Inset showsschematic and SEM image of device. Reproduced with permission from Roy et al [102] Copyright2017, John Wiley & Sons. (d) Transmittance of all-CVD MoS2/graphene photodetector. Optical imagehighlights transparency (inset). Reproduced with permission under CC-BY 4.0 from De Fazio et al [104],2015 ACS Publications. (e) Schematic of ionic polymer gated WS2-graphene photodetector (upper) andspectral responsivity (lower). (f) Optical bandwidth of device in (e) extends to 1.5 kHz. Inset showseye diagram obtained at 2.9 kbit/s. Reproduced with permission under CC-BY 4.0 from Mehew et al[28], 2017 John Wiley & Sons.

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in a reduction in the channel noise by 6 to 8 orders of magnitude. This allowed them to demonstrate anumber-resolved photo counter capable of determining the poissonian emission statistic of an LED,figure 7c. Several groups have extended these prototypes to all large-area CVD versions [103,104],without a loss of the significant gain mechanism. De Fazio et al used monolayers of graphene and MoS2

to realise a semitransparent (92% transmittance at λ = 642 nm) and flexible photodetector suitable forwearable applications, figure 7d. To extend the spectral range into the NIR different 2D materials havebeen used as the photoactive material including Bi2Te3 (Eg ∼ 0.3 eV) [130] and MoTe2 (Eg ∼ 1.1 eV)[106]. However, in these cases the responsivity is much lower than reported for MoS2, typically lessthan 103 A/W.

The slow temporal response of these heterostructure photodetectors limits their use to steadystate imaging applications as the presence of long-lived charge traps that provide the extremephoto-sensitivity comes at the expense of operational speed. Recently, Mehew et al fabricatedgraphene–tungsten disulphide (WS2) heterostructure photodetectors encapsulated in the ionic polymerLiClO4-PEO, figure 7e [28]. In this structure, WS2 is the light absorbing layer whilst the LiClO4-PEOacts as a flexible and transparent top gate. Interestingly this device can be operated at bandwidthsup to 1.5 kHz whilst maintaining a gain in excess of 106 (figure 7f) without the need for gate pulsingor implementation of a photodiode structure. Highly mobile ions of the ionic polymer screen chargetraps present in the device resulting in sub-millisecond response times and a responsivity of 106 A/W[28]. Lu et al [105] developed a vacuum annealing process to eliminate the trap states formed at theinterface between graphene and GaSe nanosheets.

In place of a sensitising material, inserting a thin tunnel barrier between two graphene sheetscan give rise to a large gain mechanism as photoexcited hot carriers generated in the top layer tunnelinto the bottom layer. Liu et al [107] reported an ultra-broadband spectral response with responsivity∼ 1 A/W in the MIR increasing to 103 A/W in the visible using a 5-nm-thick Ta2O5 layer sandwichedbetween two CVD-grown graphene monolayers. The operating bandwidth of this device is 35 Hz andwith a NEP of 1× 10−11 WHz−1/2 it is possible to estimate a LDR of 15 dB.

6. Summary and future outlook

The development of ultra-thin, flexible and transparent photodetectors is ongoing. Graphene andother two-dimensional materials have enabled a new class of devices to be developed. This articlepresents an overview of the photodetector technologies based on chemically-functionalised grapheneand hybrid structures such as graphene/QDs interfaces and graphene/TMDs heterostructures.

Pristine graphene has a broadband absorption and fast response dominated by hot-carrierdynamics. The limited responsivity of these devices is related to the low absorption of single-layergraphene and to the fast relaxation time which does not allow carriers multiplication. To enhance theabsorption and to improve the responsivity, chemical functionalisation has been used to modify theproperties of pristine graphene. Although functionalised graphene PDs show a small improvementin responsivity and a drop in operating speed (bandwidth) compared to pristine graphene, chemicalfunctionalisation allows photodetection from UV to THz wavelengths. Furthermore, quenching ofPTE results in an increase of the LDR of such detectors. Chemical functionalisation also allows thecreation of solution-processed materials, such as GO, with clear advantages in scalability.

Improvements in responsivity and operating bandwidth have been achieved by combininggraphene with other materials to form hybrid photodetectors. Photodetectors based on hybridinterfaces of graphene with QDs, semiconductors to include atomically thin TMDs, perovskitesand organic crystals offer improvements in responsivity and high gain owing to the photogating effectwhich enables charge multiplication in the graphene channel (charge recirculation). The majorityof these devices have a limited LDR due to the charge relaxation time which quickly saturates theavailable states for photoexcitation, leading to a drop in responsivity with incident optical power. Insome architectures, however, this effect is compensated by the low NEP, giving both high LDR andhigh responsivity. Therefore, a thorough investigation of charge trapping mechanisms is necessary to

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design a high performance PD. The speed of these devices is limited by the charge trapping times withreported operating bandwidths between ∼ 1 Hz and ∼ 10 kHz. Other limitations of biased graphenedetectors are the high noise levels and power consumption given the large dark current present.

Future developments of atomically thin PDs will focus on the optimization of the responsivityand bandwidth for a given application. For instance, as quantum technologies become more important,the ability to manipulate single quanta of light is a priority and single-photon detectors will haveto be integrated in optical communications systems and computation circuits at room temperature.Current state-of-the-art commercial single photon detectors have a dark count of 25 electrons/s, whichtranslates to a minimum detectable power of 0.14 fW (NEP) and a bandwidth exceeding 30 MHz[131]. This corresponds to a detectivity of 1× 1017 Jones assuming a 10 µm2 active area. Currentlysuch performance has been approached in graphene-based detectors operating at low temperature(T ∼ 100 K) [102]. Atomically-thin single-photon detectors, operating at room temperature, will allow amuch faster integration into electronics and computation systems. Furthermore, thermal managementof such devices will be facilitated by the very small footprint and low-energy consumption [132].

Although high responsivity is an important parameter, in applications where high levels ofillumination are present it is more important to have a large LDR, in order to avoid saturating a detectorundergoing abrupt changes in radiation intensity. Such scenarios include surveillance or monitoringin harsh environments such as space or inside a nuclear reactor. For instance, recent efforts in nuclearfusion have been focussing in igniting a plasma using high-power lasers. In this case monitoring thepower of the laser and the corresponding plasma is vital and small-footprint photodetectors couldsignificantly facilitate the operation of such reactors. FeCl3-intercalated graphene detectors have showna strong resilience when illuminated with high power densities (up to 100 MW/cm2) [14] and in harshenvironments [67]. The large saturation power is due to the removal of bottlenecks in the coolingmechanism of hot carriers in graphene, enabled by the high levels of doping induced by the FeCl3molecules. This change in carrier dynamics increases the available density of states for photoexcitationand leads to a linear response with incident power [14].

Healthcare applications will also benefit from ultra-thin, flexible photodetectors able to operateacross a wide range of wavelengths [108]. UV radiation, for example, is used for water purificationand sterilisation. Control of the levels of illumination is important to enhance the efficiency of suchtechniques, especially if deployed in remote locations not served by an electricity grid. IR to THzradiation is used to perform spectroscopy measurements for chemical analysis. The developmentof portable and disposable spectrometers will allow to perform complex chemical analysis in-situ,with evident benefits for fast and targeted response, for example, to an epidemic or environmentalpollution. For these kind of applications, spectral selectivity and high responsivity are needed. Fastresponse time is not critical as the majority of analytical techniques have a time-scale much larger thatthe response time of the PDs.

In summary, graphene-based photodetectors, whilst offering a small footprint promising fornext-generation flexible and wearable electronics, are also more energy-efficient and could providefeatures which are not available in bulk semiconductors, such as polarisation sensitivity [133]and strain-tunable response [134]. The family of more than 2000 layered materials offer a wealthof possibilities for the realisation of novel optoelectronic devices which can integrate multiplefunctionalities in a single active material.

Author Contributions: A.De-S. and J.D.M. collected and analysed the data, wrote the first draft and prepared thefigures. All authors contributed to the final revision.

Funding: M.F.C. and S.R. acknowledge financial support from: Engineering and Physical Sciences ResearchCouncil (EPSRC) of the United Kingdom, projects EP/M002438/1, EP/M001024/1, EPK017160/1, EP/K031538/1,EP/J000396/1; the Royal Society, grant title "Room temperature quantum technologies" and "Wearable graphenephotovolotaic"; Newton fund, Uk-Brazil exchange grant title "Chronographene" and the Leverhulme Trust,research grants "Quantum drums" and "Quantum revolution". J.D.M. acknowledges financial support from theEngineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre forDoctoral Training in Metamaterials, Grant No. EP/L015331/1.

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Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript (in alphabetic order):

CNP Charge neutrality pointEQE External quantum efficiencyFET Field-effect transistorFG FluorographeneFLG Few-layer grapheneGO Graphene oxidehBN Hexagonal boron nitrideIQE Internal quantum efficiencyIR infra-redLDR Linear dynamic rangeLED Light-emitting diodeMIR mid infra-redNEP Noise Equivalent PowerNIR near infra-redPD(s) Photodetector(s)PG PhotogatingPSD Posision-sensitive (photo)detectorPTE Photo-thermoelectric effectPV PhotovoltaicQD(s) Quantum dot(s)rGO reduced graphene oxideSPCM Scanning-photocurrent map(ing)TMD(s) Transition-metal dichalcogenide(s)UV ultra-violetvdW van der Waals

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